1. Field of the Invention
The present invention relates to a gamma correcting circuit for use with a liquid-crystal television receiver or the like, for effecting inverse gamma correction on a video signal and outputting the corrected video signal.
2. Description of the Prior Art
There have been proposed liquid-crystal television receivers which incorporate liquid-crystal display units, rather than cathode-ray tubes, for displaying video images. For displaying video images on liquid-crystal display units based on a video signal such as a television signal intended to be supplied to cathode-ray tubes, it is necessary to process such a video signal because of the difference between the characteristics of liquid-crystal display units and cathode-ray tubes. Specifically, a video signal which has been gamma-corrected according to the illuminance characteristics of the cathode-ray tubes needs to be subjected to inverse gamma correction, thus removing the nonlinearity of the gamma-corrected video signal. After the inverse-gamma-corrected video signal is amplified, it is processed according to the voltage vs. transmittance (V-T) characteristics of the liquid-crystal display units.
FIG. 1 of the accompanying drawings shows a conventional gamma correcting circuit disclosed in Japanese laid-open patent publication No. 260976/90. As shown in FIG. 1, the conventional gamma correcting circuit has an input terminal 1 for introducing a video signal X and an output terminal 2 for outputting a corrected video signal. A buffer transistor Q9 has a collector connected to a voltage power supply VCC, a base connected to the input terminal 1, and an emitter connected to ground through a regulated constant current supply I4. A buffer 3 comprises a buffer amplifier for generating a reference voltage, at a pedestal level of the video signal X, in response to a burst gate pulse P that is applied at a burst period of the video signal X. The buffer 3 has an input terminal connected to the emitter of the transistor Q9 and an output terminal connected to ground through a capacitor C. The capacitor C serves to keep the potential of the output terminal of the buffer amplifier 3 at the reference voltage when the burst gate pulse P is not applied to the buffer amplifier 3.
The conventional gama correcting circuit also has differential amplifiers A, B, C. The differential amplifier A is mainly composed of a differential pair of transistors Q1, Q2 each comprising an NPN transistor. The transistor Q1 has a collector connected to the voltage power supply VCC through a resistor R1, a base connected to the emitter of the transistor Q9, and an emitter connected to a terminal of a constant current supply I1 through a resistor R2. The regulated constant current supply I1 has the other terminal grounded. The transistor Q2 has a collector connected to the voltage power supply VCC through a resistor R10 and also connected to the output terminal 2, an emitter connected to the terminal of the constant current supply I1 through a resistor R3, and a base connected to the junction between the buffer amplifier 3 and the capacitor C.
The differential amplifier B is mainly composed of a differential pair of transistors Q3, Q4 each comprising an NPN transistor. The transistor Q3 has a collector connected to the output terminal 2, a base connected to the emitter of the transistor Q9, and an emitter connected to a terminal of a variable current supply I5 through a resistor R4. The variable current supply I5 has the other terminal grounded. The transistor Q4 has a collector connected to the collector of the transistor Q1, a base connected to the output terminal of the buffer amplifier 3, and an emitter connected to the terminal of the variable constant current supply I5 through a resistor R5. The variable current supply I5 is connected through a control terminal 4 to a variable resistor RA, so that the value of a current supplied by the variable current supply I5 can be varied when the resistance of the variable resistor RA is adjusted.
The differential amplifier C is mainly composed of a differential pair of transistors Q7, Q8 each comprising an NPN transistor. The transistor Q7 has a collector connected to the collector of the transistor Q1, a base connected to the emitter of the transistor Q9, and an emitter connected to a terminal of a variable current supply I6 through a resistor R8. The variable current supply I6 has the other terminal grounded. The transistor Q8 has a collector connected to the output terminal 2, a base connected to the output terminal of the buffer amplifier 3, and an emitter connected to the terminal of the variable constant current supply I6 through a resistor R9. The variable current supply I6 is connected through the control terminal 4 to the variable resistor RA, so that the value of a current supplied by the variable current supply I6 can be varied when the resistance of the variable resistor RA is adjusted.
Operation of the conventional gamma correcting circuit will be described below also with reference to FIGS. 2 through 4 of the accompanying drawings. When a video signal X is supplied to the input terminal 1, a burst gate pulse P synchronous with a burst period of the video signal X is applied to the buffer amplifier 3. In response to the burst gate pulse P, the buffer amplifier 3 generates a constant reference voltage at the pedestal level of the video signal X. At this time, the capacitor C is charged up to the reference voltage. The constant voltage is applied from the buffer amplifier 3 or the capacitor C to the bases of the transistors Q2, Q4, Q8. The video signal X supplied to the input terminal 1 is amplified by the differential amplifiers A, B, C. It is assumed that the differential amplifiers A, B, C have respective input vs. output characteristics indicated by solid-line curves A1, B1, C1, respectively, shown in FIGS. 2 and 3. The conventional gamma correcting circuit outputs, through its output terminal 2, the sum of an output signal (indicated by the solid-line curve A1 shown in FIG. 2) from the differential amplifier A and the sum (indicated by the solid-line curve Y1 shown in FIG. 3) of output signals from the differential amplifiers B, C, i.e., the sum (indicated by the solid-line curve S1 shown in FIG. 4) of the output signals from the differential amplifiers A, B, C.
When the variable resistor RA is adjusted to vary the values of currents supplied by the variable current supplies I5, I6, the conventional gamma correcting circuit operates as follows: When the variable resistor RA is adjusted to reduce the resistance thereof, the input vs. output characteristics of the differential amplifiers B, C vary as indicated by broken-line curves B1a, C1a, respectively, in FIG. 3, varying their output clipping levels. The sum of the output signals from the differential amplifiers B, C now varies as indicated by a broken-line curve Ya in FIG. 3. The sum of the sum of the output signals from the differential amplifiers B, C and the output signal from the differential amplifier A now varies as indicated by a broken-line curve S1a in FIG. 4. When the variable resistor RA is adjusted to increase the resistance thereof, the input vs. output characteristics of the differential amplifiers B, C vary as indicated by broken-line curves B1b, C1b, respectively, in FIG. 3, varying their output clipping levels. The sum of the output signals from the differential amplifiers B, C now varies as indicated by a dotted-line curve Yb in FIG. 3. The sum of the sum of the output signals from the differential amplifiers B, C and the output signal from the differential amplifier A now varies as indicated by a broken-line curve S1b in FIG. 4. Therefore, when the variable resistor RA is adjusted, the input vs. output characteristics of the conventional gamma correcting circuit vary in the directions indicated by the arrow 5 in FIG. 4, thereby gamma-correcting the video signal X.
The conventional gamma correcting circuit has been disadvantageous in that the range in which the output signal from the output terminal 2 is variable is limited, and hence the video signal X may need to be adjusted in level to produce a desired output signal.